child pornography.. problem

[woody]

if they're using a mobile, block the browser. ok you'll lose anynone using the same one but it'd be worth it, just make it redirect to another page so he doesnt even realise its blocked he'll just think the sites down

[EpIcInCoGnItO]

As people have said here you should use registration to ban that user account permanently. And also you can you file verification method before display.

[Unfathomable]

Putting up a registration and email validation is the best method. Also, buy a dedicated server if you can.

[GumSlone]

for file sharing site you have to add file moderators to your site there is no way around, most of big file sharing sites already done it, ban ip's or even countries from uploading specific files.

Added after 5 minutes:

ip ban will not work because mobile is having dynamic IP

ip block works just replace the last value of ip and block first 3 blocks like 255.255.255.

just found this Image Nudity Filter (image filter, skin, score, probability, analysis, php filter) - PHP Classes

try it

this filter is good if you want to block all the adult content, but it might be a problem with videos, because you will need to extract several frames from video.

as for me, file moderation is still the best way.